Ultrasound Testing

ABSTRACT

An apparatus for imaging structural features below the surface of an object, the apparatus comprising: a transmitter unit configured to transmit a sound pulse at the object; a receiver unit configured to receive reflections of sound pulses transmitted by the transmitter unit from the object; a signal processing unit configured to: analyse one or more signals received by the receiver unit from the object; recognise, in the one or more signals, a reflection that was caused by a first structural feature and a reflection that was caused by a second structural feature that is located, in the object, at least partly behind the first structural feature; and associate each recognised reflection with a relative depth in the object at which the reflection occurred; and an image generation unit configured to generate an image that includes a representation of the first and second structural features in dependence on the recognised reflections and their relative depths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to United Kingdom Patent ApplicationNo. 1314483.7 entitled Ultrasound Testing, which was filed on Aug. 13,2013. The disclosure of the foregoing application is incorporated hereinby reference in its entirety.

BACKGROUND

This invention relates to an apparatus for imaging structural featuresbelow an object's surface. The apparatus may be particularly useful forimaging sub-surface material defects such as delamination, debonding andflaking.

Ultrasound is an oscillating sound pressure wave that can be used todetect objects and measure distances. A transmitted sound wave isreflected and refracted as it encounters materials with differentacoustic impedance properties. If these reflections and refractions aredetected and analysed, the resulting data can be used to describe theenvironment through which the sound wave travelled.

Ultrasound may be used to detect and decode machine-readable matrixsymbols. Matrix symbols can be directly marked onto a component bymaking a readable, durable mark on its surface. Commonly this isachieved by making what is in essence a controlled defect on thecomponent's surface, e.g. by using a laser or dot-peening. Matrixsymbols can be difficult to read optically and often get covered by acoating like paint over time. The matrix symbols do, however, often havedifferent acoustic impedance properties from the surrounding substrate.U.S. Pat. No. 5,773,811 describes an ultrasound imaging system forreading matrix symbols that can be used to image an object at a specificdepth. A disadvantage of this system is that the raster scanner has tobe physically moved across the surface of the component to read thematrix symbols. U.S. Pat. No. 8,453,928 describes an alternative systemthat uses a matrix array to read the reflected ultrasound signals sothat the matrix symbol can be read while holding the transducerstationary on the component's surface.

Ultrasound can also be used to identify other structural features in anobject. For example, ultrasound may be used for non-destructive testingby detecting the size and position of flaws in an object. The ultrasoundimaging system of U.S. Pat. No. 5,773,811 is described as being suitablefor identifying material flaws in the course of non-destructiveinspection procedures. The system is predominantly intended for imagingmatrix symbols so it is designed to look for a “surface” below anylayers of paint or other coating on which the matrix symbols have beenmarked. It is therefore designed to operate at specific depths, whichcan be controlled by gating the received signal. The ultrasound systemof U.S. Pat. No. 5,773,811 also uses a gel pack to couple ultrasoundenergy into the substrate, which may make it difficult to accuratelydetermine the depth of features below the substrate's surface.

SUMMARY

There is a need for an improved apparatus for imaging structuralfeatures below the surface of an object.

According to a first embodiment of the present invention there isprovided an apparatus for imaging structural features below the surfaceof an object, the apparatus comprising a transmitter unit configured totransmit a sound pulse at the object, a receiver unit configured toreceive reflections of sound pulses transmitted by the transmitter unitfrom the object, a signal processing unit configured to: analyse one ormore signals received by the receiver unit from the object; recognise,in the one or more signals, a reflection that was caused by a firststructural feature and a reflection that was caused by a secondstructural feature that is located, in the object, at least partlybehind the first structural feature; and associate each recognisedreflection with a relative depth in the object at which the reflectionoccurred, and an image generation unit configured to generate an imagethat includes a representation of the first and second structuralfeatures in dependence on the recognised reflections and their relativedepths.

The receiver unit may be configured to receive a signal comprisingmultiple reflections of a single transmitted sound pulse.

The receiver unit may configured to receive the multiple reflectionswhile in a stationary position with respect to the object.

The apparatus may be configured to gate a signal received by thereceiver unit in such a way that the signal passed to the signalprocessing unit for analysis comprises the multiple reflections.

The apparatus may be configured to apply an adjustable time-gate to thereceived signal.

The apparatus may be configured such that the time-gate is adjustable bya user.

The apparatus may comprise a pulse generation unit configured to form apulse having a particular shape for transmission as a sound pulse by thetransmitter unit.

The signal processor may comprise a match filter configured to recognisea pulse having the particular shape in the received signal.

The apparatus may have a plurality of particular pulse shapes availableto it, the pulse generation unit may be configured to assess anavailable pulse shape against a performance criterion and select thatpulse shape as a candidate for transmission in dependence on thatassessment.

The apparatus may be configured such that the particular shape isselectable by the user.

The signal processor may be configured to associate each recognisedreflection with a relative depth that is determined in dependence on atime that the recognised reflection took to travel from the structuralfeature that caused the reflection to the receiver unit.

The signal processor may be configured to associate each recognisedreflection with a maximum amplitude and to adjust that maximum amplitudein dependence on the relative depth associated with that reflection.

The transmitter unit may be configured to transmit a series of soundpulses into the object and the image generation unit being configured togenerate an image, for each sound pulse in the series, in dependence onthe reflections of that sound pulse that are recognised by the signalprocessing unit.

The image generation apparatus may be configured to generate an image,for a sound pulse in the series, in dependence an image generated for asound pulse that preceded it in the series.

The image generation apparatus may be configured to generate an image inwhich the first and second structural features are represented atpositions in the image that reflect their relative depths below thesurface of the object.

The apparatus may comprise a handheld device that comprises at least thetransmitter and receiver units.

The apparatus may comprise either an integrated display for displayingthe image or may be configured to output the image to a handheld displaydevice.

The image generation unit may be configured to identify a reflectionhaving a maximum amplitude that is below a threshold value and assign apixel in the image that corresponds to the identified reflection apredetermined value instead of the reflection's maximum amplitude.

The image generation unit may be configured to assign the pixel apredetermined value that is higher than the reflection's maximumamplitude.

The apparatus may comprise a dry coupling medium.

The apparatus may comprise a matrix array for transmitting and receivingsound pulses.

According to a second embodiment of the invention, there is provided amethod for imaging structural features below the surface of an objectcomprising: transmitting a sound pulse at the object; receiving, fromthe object, reflections of sound pulses transmitted at the object;recognising, in one or more signals received from the object, areflection of a transmitted time pulse that was caused by a firststructural feature and a reflection of a transmitted time pulse that wascaused by a second structural feature that is located, in the object, atleast partly behind the first structural feature; associating eachrecognised reflection with a relative depth in the object at which thereflection occurred; and generating an image that includes arepresentation of the first and second structural features in dependenceon the recognised reflections and their relative depths.

DESCRIPTION OF DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings. In the drawings:

FIG. 1 shows an example of an imaging apparatus and an object;

FIGS. 2 a to c show different examples of an imaging apparatus and anobject;

FIG. 3 shows an example of the functional blocks comprised in an imagingapparatus;

FIGS. 4 a to c show examples of an ultrasound signal and a correspondingmatch filter;

FIGS. 5 a and b show example images of an object;

FIG. 6 shows an example of an imaging apparatus;

FIG. 7 shows an example of the functional blocks implemented by an FPGA;

FIG. 8 shows an example of imaging apparatus; and

FIG. 9 shows an example of the steps involved in an imaging method.

DETAILED DESCRIPTION

An apparatus for imaging structural features below the surface of anobject is shown in FIG. 1. The apparatus, shown generally at 101,comprises a transmitter unit 102, a receiver unit 103, a signalprocessing unit 104 and an image generation unit 105. In one example thetransmitter and receiver unit may be implemented by a single ultrasoundtransducer. The transmitter and receiver units are shown next to eachother in FIG. 1 for ease of illustration only. In a practicalrealisation of a transducer it is likely that the transmitter andreceiver units will be implemented as layers one on top of the other.The transmitter unit is suitably configured to transmit a sound pulsehaving a particular shape at the object to be imaged 106. The receiverunit is suitably configured to receive reflections of transmitted soundpulses from the object.

There are two sub-surface structural features 107, 108 in the object.One of the structural features 108 is located partly behind the otherfeature 107. Generally, the second structural feature is located fartheralong the path of the transmitted pulse than the first structuralfeature. Another way of looking at it is that both features lie on astraight line normal to a plane that contacts the object's surface; thesecond structural feature 108 is located farther along the straight linefrom the plane than the first structural feature 107. Example scenariosare illustrated in FIGS. 2 a to c. In these examples the apparatus has aflat base which can be considered to approximate the “plane”, and thesound pulses are transmitted in a direction normal to that surface, sotheir path can be considered to approximate the “straight line”. (Inpractice, the base of the apparatus need not be flat, nor are the pulsesnecessarily transmitted in a direction normal to the base of theapparatus, but it is a useful comparison for visualising thearrangement).

FIGS. 2 a to c also include examples in which the structural featuresare not necessarily contained within the solid body of the object. Thefeatures could be contained within a hole, depression or other hollowsection. Such features are considered to be “in” the object and “below”its surface for the purposes of this description because they lie on thepath of the sound pulses as they travel from the apparatus through theobject. This model of reflection and propagation is most likely to occurin solid sections of the object for two reasons: (i) ultrasound isattenuated strongly by air; and (ii) air-object boundaries tend to showa big impedance mismatch, so that majority of ultrasound encounteringsuch a boundary will be reflected.

Structural features that are located behind other features are generally“invisible” to existing imaging systems. Signal processing unit 104,however, is configured to recognise reflections caused by bothstructural feature 107 and 108 in the signals received by receiver unit103. The signal processing unit is also configured to associate eachrecognised reflection with a relative depth, which represents the depthin the object at which the reflection occurred, i.e. the depth of thestructural feature that caused the reflection. This information enablesimage generation unit 105 to generate an image that represents both thefirst and second structural features. The image may be displayed for anoperator, enabling sub-surface features to be detected and evaluated.This enables the operator to see “into” the object and can providevaluable information on sub-surface material defects such asdelamination, debonding and flaking.

There are a number of ways in which the apparatus may be configured toidentify reflections from structural features that are obscured by otherfeatures closer to the surface. One option is to use differenttransmitted sound pulses to gather information on each structuralfeature. These sound pulses might be different from each other becausethey are transmitted at different time instants and/or because they havedifferent shapes or frequency characteristics. The sound pulses might betransmitted at the same location on the object's surface or at differentlocations. This may be achieved by moving the apparatus to a differentlocation or by activating a different transmitter in the apparatus. Ifchanging location alters the transmission path sufficiently a soundpulse might avoid the structural feature that, at a different location,had been obscuring a feature located farther into the object. Anotheroption is to use the same transmitted sound pulse to gather informationon the different structural features. This option uses differentreflections of the same pulse and is described in more detail below. Theapparatus may implement any or all of the options described above andmay combine data gathered using any of these options to generate asub-surface image of the object. The image may be updated and improvedon a frame-by-frame basis as more information is gathered on thesub-surface structural features.

In one embodiment the apparatus uses the same transmitted sound pulse togather information on structural features that are positioned one on topof the other in the object.

The sound pulse suitably has a particular shape and is transmitted bythe transmitter unit. The signal received by the receiver unit willtypically include a number of reflections of the transmitted soundpulse. These reflections are caused by features of the materialstructure below the object's surface. Reflections are caused byimpedance mismatches between different layers of the object, e.g. amaterial boundary at the join of two layers of a laminated structure.Often only part of the transmitted pulse will be reflected and aremainder will continue to propagate through the object (as shown inFIGS. 2 a to c). The remainder may then be wholly or partly reflected asit encounters other features in the material structure.

The signal received by the receiver unit at a single location on theobject's surface is likely to contain two or more successive reflectionsof the same transmitted pulse. Each reflection represents a differentstructural feature. Pre-existing imaging devices tend to discard theselater reflections because they are not of interest. An apparatus forcapturing matrix codes, for example, will typically only be interestedone reflection: the reflection off the matrix symbol. When detectingsub-surface defects, it is preferable to capture multiple reflections ofthe same transmitted pulse; this enables surface defects located belowother structural features in the object to be identified.

The signal processing unit is suitably configured to analyse thereceived signal to find sections of the signal that representreflections or echoes of the transmitted pulse. The pulses preferablyhave a known shape so that the signal processor is able to identifytheir reflections. The signal processing unit is suitably configured torecognise two or more reflections of a single transmitted pulse in thereceived signal. The signal processing unit is also configured toassociate each reflected pulse with a relative depth, which could be,for example, the depth of the structural feature relative to transmitterand/or receiver, the depth of the structural feature relative thesurface of the object, or the depth of the feature relative to anotherstructural feature in the object. Normally the relative depth will bedetermined from the time-of-flight of the reflection (i.e. the time thereflection took to return to the apparatus) and so it represents thedistance between the structural feature and the receive unit.

An example of the functional blocks comprised in one embodiment of theapparatus are shown in FIG. 3.

In this example the transmitter and receiver are implemented by anultrasound transducer 301, which comprises a matrix array of transducerelements 312. The transducer elements transmit and/or receive ultrasoundwaves. The matrix array may comprise a number of parallel, elongatedelectrodes arranged in an intersecting pattern; the intersections formthe transducer elements. The transmitter electrodes are connected to thetransmitter module 302, which supplies a pulse pattern with a particularshape to a particular electrode. The transmitter control 304 selects thetransmitter electrodes to be activated. The number of transmitterelectrodes that are activated at a given time instant may be varied. Thetransmitter electrodes may be activated in turn, either individually orin groups. Suitably the transmitter control causes the transmitterelectrodes to transmit a series of sound pulses into the object,enabling the generated image to be continuously updated. The transmitterelectrodes may also be controlled to transmit the pulses using aparticular frequency. The frequency may be between 100 kHz and 30 MHz,preferably it is between 1 and 15 MHz and most preferably it is between2 and 10 MHz.

The receiver electrodes sense sound waves that are emitted from theobject. These sound waves are reflections of the sound pulses that weretransmitted into the object. The receiver module receives and amplifiesthese signals. The signals are sampled by an analogue-to-digitalconverter. The receiver control suitably controls the receiverelectrodes to receive after the transmitter electrodes have transmitted.The apparatus may alternately transmit and receive. In one embodimentthe electrodes may be capable of both transmitting and receiving, inwhich case the receiver and transmitter controls will switch theelectrodes between their transmit and receive states. There ispreferably some delay between the sound pulses being transmitted andtheir reflections being received at the apparatus. The apparatus mayinclude a dry coupling layer to provide the delay needed for theelectrodes to be switched from transmitting to receiving. Any delay maybe compensated for when the relative depths are calculated. The drycoupling layer preferably provides low damping of the transmitted soundwaves.

Each transducer element may correspond to a pixel in the image. In otherwords, each pixel may represent the signal received at one of thetransducer elements. This need not be a one-to-one correspondence. Asingle transducer element may correspond to more than one pixel andvice-versa. Each image may represent the signals received from onepulse. It should be understood that “one” pulse will usually betransmitted by many different transducer elements. These versions of the“one” pulse might also be transmitted at different times, e.g. thematrix array could configured to activate a “wave” of transducerelements by activating each line of the array in turn. This collectionof transmitted pulses can still considered to represent “one” pulse,however, as it is the reflections of that pulse that are used togenerate a single image of the sample. The same is true of every pulsein a series of pulses used to generate a video stream of images of thesample.

The pulse selection module 303 selects the particular pulse shape to betransmitted. It may comprise a pulse generator 313, which supplies thetransmitter module with an electronic pulse pattern that will beconverted into ultrasonic pulses by the transducer. The pulse selectionmodule may have access to a plurality of predefined pulse shapes storedin memory 314. The pulse selection module may select the pulse shape tobe transmitted automatically or based on user input. The shape of thepulse may be selected in dependence on the type of structural featurebeing imaged, its depth, material type etc. In general the pulse shapeshould be selected to optimise the information that can be gathered bythe signal processor 305 and/or improved by the image enhancement module310 in order to provide the operator with a quality image of the object.

In one example a match filter that the signal processor uses torecognise reflections of a transmitted pulse may be selected tocorrespond to the selected pulse shape. Examples of an ultrasound signals(n) and a corresponding match filter p(n) are shown in FIGS. 4 a and brespectively. The ultrasound signal s(n) is a reflection of atransmitted pulse against air.

The aim is to select a pulse shape and corresponding match filter thatwill achieve a precise estimate of the time-of-flight of the reflectedpulse, as this indicates the depth of the structural feature thatreflected the pulse. The absolute values of the filtered time series(i.e. the absolute of the output of the match-filter) for ultrasoundsignal s(n) and corresponding match filter p(n) are shown in FIG. 4 c.The signal processor estimates the time-of-flight as the time instantwhere the amplitude of the filtered time series is at a maximum. In thisexample, the time-of-flight estimate is at time instant 64. If thesignal contains a lot of noise, however, this may cause other timeinstants to produce a higher value. The ideal output of the filter, toobtain the most precise time-of-flight estimate, would be a deltafunction with all samples having zero-amplitude apart from that at timeinstant 64 (for this case). Since this is not realisable in practice,the aim is to select pulse shapes and match filters to achieve a goodmargin between the amplitude of the main lobe and the amplitude of anyside lobes.

The apparatus may determine an optimum pulse shape by obtaining areflection of each transmitted pulse against a particular material(which in one example may be air), filtering it using the appropriatematched filter and determining how the different pulse shapes performedaccording to the following criteria:

-   -   The ratio between the main lobe and side lobe amplitudes (see        FIG. 4 c). This criterion does not consider the signal to noise        ratio (SNR) and does risk selecting pulse shapes that could be        lost in noise at the receiver.    -   The difference between the main lobe and side lobe amplitudes,        normalized with root-mean-square (RMS) of the matched filter        coefficients. This criterion may penalize signals with a low        SNR.    -   The SNR, i.e. the amplitude of the main lobe divided RMS of the        filter coefficients.    -   The bandwidth of the signal. This criterion is founded on the        fact that the filtered output of signals tends to approach a        delta-function as the bandwidth increases. The bandwidth is        calculated using the derivative of a smoothing spline        approximation of the ultrasound signal.    -   The variance of the estimated time-of-flight. This is directly        related to the signal bandwidth.    -   The main lobe amplitude divided by the average amplitude of all        side lobes. This criterion also risk of selecting pulse shapes        with a low SNR.

The operator may be able to choose between pulse shapes that aredetermined to be optimum by the apparatus.

The signal processor is configured to extract relevant information fromthe received ultrasound signals. The signal is suitably time-gated sothat the signal processor only processes reflections from depths ofinterest. The time-gating may be adjustable, preferably by a user, sothat the operator can focus on the depths he is interested in. The depthrange is preferably 0 to 20 mm, and most preferably 0 to 15 mm.

The signal processor is preferably capable of recognising multiple peaksin each received signal. It may determine that a reflection has beenreceived every time that the output of the match filter exceeds apredetermined threshold. It may identify a maximum amplitude for eachacknowledged reflection.

In some embodiments the apparatus may be configured to accumulate andaverage a number of successive samples in the incoming sample (e.g. 2 to4) for smoothing and noise reduction before the filtering is performed.The signal processor is configured to filter the received signals usinga match filter, as described above, to accurately determine when thereflected sound pulse was received at the apparatus. The signalprocessor then performs features extraction to capture the maximumamplitude of the filtered signal and the time at which that maximumamplitude occurs. The signal processor may also extract phase and energyinformation.

In one embodiment the apparatus may amplify the filtered signal beforeextracting the maximum amplitude and time-of-flight values. This may bedone by the signal processor. The amplification steps might also becontrolled by a different processor or FPGA. In one example the timecorrected gain is an analogue amplification. This may compensate for anyreduction in amplitude that is caused by the reflected pulse's journeyback to the receiver. One way of doing this is to apply a time-correctedgain to each of the maximum amplitudes. The amplitude with which a soundpulse is reflected by a material is dependent on the qualities of thatmaterial (for example, its acoustic impedance). Time-corrected gain can(at least partly) restore the maximum amplitudes to the value they wouldhave had when the pulse was actually reflected. The resulting imageshould then more accurately reflect the material properties of thestructural feature that reflected the pulse. The resulting image shouldalso more accurately reflect any differences between the materialproperties of the structural features in the object. The signalprocessor may be configured to adjust the filtered signal by a factorthat is dependent on its time-of-flight.

The image construction module may construct a number of different imagesof the object using the information gathered by the signal processor.Any of the features extracted by the signal processor from the receivedsignal may be used to generate an image. Typically the images representthe depth associated with a reflection received at a given point on theobject's surface and the energy or amplitude of that reflection. Sincethe signal processor can identify multiple reflections of a given soundpulse at a particular point on the object's surface, the image will showsub-surface structural features located immediately below one anotherfrom the operator's perspective. The image construction module mayassociate each pixel in an image with a particular location on thereceiver surface, so that each pixel represents a reflection that wasreceived at the pixel's associated location.

The image construction module may be able to generate an image from theinformation gathered using a single transmitted pulse. The imageconstruction module may update that image with information gathered fromsuccessive pulses. The image construction module may generate a frame byaveraging the information for that frame with one or more previousframes so as to reduce spurious noise. This may be done by calculatingthe mean of the relevant values that form the image.

The image enhancement module 310 enhances the generated images to reducenoise and improve clarity. The image processing module may process theimage differently depending on the type of image. (Some examples areshown in FIGS. 5 a and b and described below.) The image enhancementmodule may perform one or more of the following:

-   -   Time Averaging. Averaging over the current and the previous        frame may be performed by computing the mean of two or more        successive frames for each point to reduce spurious noise.    -   Background compensation. The background image is acquired during        calibration by transmitting a sound pulse at air. All the        reflected pulse-peaks toward air are converted to the range [0,        1]. This is a digital compensation and most values will be        converted to 1 or nearly 1. The ultrasound camera (e.g. the        ultrasound transducer in the example of FIG. 3) inherently has        some variations in performance across its surface that will        affect the time and amplitude values extracted by the signal        processor. To compensate for this, images obtained during normal        operation are divided by the background image.    -   Signal envelope estimation. An analytic representation of the        background compensated signal may be created as the sum of the        signal itself and an imaginary unit times the Hilbert transform        of the signal. The analytic signal is a complex signal, from        which the signal envelope can be extracted as the magnitude of        the analytic signal and used in further processing. Generation        of low-amplitude mask. This process may be used particularly for        generating 3D images. A mask covering pixels that have amplitude        values lower than a threshold is created. (This threshold may be        lower than the threshold value for the thresholding described        below.) A filter such as a 3×3 maximum filter is then used on        the resulting mask to close small holes.    -   Thresholding: A threshold percentage can be specified so that        low amplitude values do not clutter the image. In some        embodiments this may be set by the operator. A threshold value        is calculated from the percentage and the total range of the        amplitude values. Parts the image having an amplitude value        lower than this threshold are truncated and set to the threshold        value. A threshold percentage of zero means that no thresholding        is performed. The purpose of the thresholding is to get a        cleaner visualization of the areas where the amplitude is low.    -   Normalization: The values are normalized to the range 0-255 to        achieve good separation of colours when displayed. Normalization        may be performed by percentile normalization. Under this scheme        a low and a high percentile can be specified, where values        belonging to the lower percentile are set to 0, values belonging        to the high percentile are set to 255 and the range in between        is scaled to cover [0, 255]. Another option is to set the colour        focus directly by specifying two parameters,        colorFocusStartFactor and colorFocusEndFactor, that define the        start an end points of the range. The values below the start        factor are set to 0, values above the end factor is set to 255        and the range in between is scaled to cover [0, 255].    -   Filtering. Images may be filtered to reduce spurious noise. Care        should to be taken that the resulting smoothing does not to blur        edges too much. The most appropriate filter will depend on the        application. Some appropriate filter types include: mean,        median, Gaussian, bilateral and maximum similarity.    -   Generation of colour matrix. A colour matrix is created that        specifies values from the grey-level range of the colour table        for low-amplitude areas and values from the colour range for the        remaining, higher-amplitude areas. A mask for the grey level        areas may be obtained from an eroded version of the        low-amplitude mask. (The erosion will extend the mask by one        pixel along the edge between grey and colour and is done to        reduce the rainbow effect that the visualization would otherwise        create along the edges where the pixel value goes from the grey        level range to the colour range.)

Some examples of images generated by an apparatus according to oneembodiment are shown in FIGS. 5 a and b. The A-scan 501 is astraightforward plot of amplitude against depth for a particularlocation on the object's surface. Depth is calculated from thetime-of-flight information. The peaks represent structural features thatreflected the sound pulses. The cross hairs 503,504 designate the x,ylocation that is represented by the A-scan. The horizontal slidebar 502sets the threshold percentage.

Image 505 is a two-dimensional representation of the image formed fromreflections received across the matrix array. It is similar to whatmight be generated by a system for imaging matrix symbols. Iteffectively displays a sub-surface layer of the object. The example inFIG. 5 a represents time-of-flight, i.e. each pixel is allocated acolour according to the relative depth associated with the largestreflection received at the corresponding location on the object'ssurface. Other extracted features might also be imaged, e.g. amplitudeor signal energy.

The B-scan is comprised of two separate two-dimensional images thatrepresent a vertical view (y,z) 506 and a horizontal view (x,z) 507. Thevertical and horizontal views image “into” the object. The cross hairs503, 504 determine where the “slice” through the plan view 505 is taken.These views represent the sound energy reflected at different depths inthe object. The upper and lower gates 508, 509 are used to set the upperand lower bounds for time gating the incoming signals. The operator maybe able to achieve a greater colour contrast between structural featuresof interest by adjusting the gates to focus on the depth of interest.The operator may also select only a certain depth area to inspect byadjusting the gates.

FIG. 5 b shows a C-scan 510, which is 3D image. The operator may be ableto rotate and zoom-in to the 3D image. The operator can select asub-surface layer of a particular thickness to view in 3D by adjustingthe time gates 508, 509.

It can be appreciated from FIGS. 5 a and b that the images give theoperator a good idea of the size, depth and position of any sub-surfacestructural features. The plan view gives the operator an overview ofwhat is below the surface. The operator can use cross hairs 503, 504 tolook at specific horizontal and vertical slices through the object. TheA-scan indicates features located below a particular point on theobject's surface. Finally the C-scan provides the operator with auser-friendly representation of what the object looks like below itssurface.

One example of a sound imaging apparatus is illustrated in FIG. 6. Theapparatus comprises a handheld device, shown generally at 601, which isconnected via a USB connection 602 to a PC 603. The connection mightalso be wireless. The handheld device comprises a transmitter unit 605,a receiver unit 606, an FPGA 607 and a USB connector 608. The USBconnection connects the handheld device to a PC 603. The functionalunits comprised within the FPGA are shown in more detail in FIG. 7. Thetime series along the bottom of the figure show the transformation ofthe received data as it is processed.

An example of a handheld device for imaging below the surface of anobject is shown in FIG. 8. The device 801 could have an integrateddisplay, but in this example it outputs images to a tablet 802. Thedevice has a matrix array 803 for transmitting and receiving ultrasoundsignals. The handheld apparatus comprises a dry coupling layer 804 forcoupling ultrasound signals into the object. The dry coupling layer alsointroduces a delay that allows time for the transducer to switch betweentransmitting and receiving. This offers a number of advantages overother imaging systems, which tend to use liquids for coupling theultrasound signals. This can be impractical in an industrialenvironment. If the liquid coupler is contained in a bladder, as issometimes used, this makes it difficult to obtain accurate depthmeasurements which is not ideal for non-destructive testingapplications.

The matrix array 803 is two dimensional so there is no need to move itacross the object to obtain an image. A typical matrix array might be 30mm by 30 mm but the size and shape of the matrix array can be varied tosuit the application. The device may be straightforwardly held againstthe object by the operator. Commonly the operator will already have agood idea of where the object might have sub-surface flaws or materialdefects; for example, a component may have suffered an impact or maycomprise one or more drill or rivet holes that could cause stressconcentrations. The device suitably processes the reflected pulses inreal time so the operator can simply place the device on any area ofinterest.

The handheld device also comprises a dial 805 that the operator can useto change the pulse shape and corresponding filter. The most appropriatepulse shape may depend on the type of structural feature being imagedand where it is located in the object. The operator views the object atdifferent depths by adjusting the time-gating via the display (see alsoFIG. 5 a, described above). Having the apparatus output to a handhelddisplay, such as tablet 802, or to an integrated display, isadvantageous because the operator can readily move the transducer overthe object, or change the settings of the apparatus, depending on whathe is seeing on the display and get instantaneous results. In otherarrangements, the operator might have to walk between a non-handhelddisplay (such as a PC) and the object to keep rescanning it every time anew setting or location on the object is to be tested.

An example of a method that may be performed to generate a sub-surfaceimage of an object is shown in FIG. 9. The method comprises transmittinga sound pulse (step 901) and receiving reflections of that transmittedpulse (step 902). The maximum amplitude and relative depth is identifiedfor each reflection (step 903). The gathered information is then used togenerate a sub-surface image of the object (step 904).

The apparatus and methods described herein are particularly suitable fordetecting debonding and delamination in composite materials such ascarbon-fibre-reinforced polymer (CFRP). This is important for aircraftmaintenance. It can also be used detect flaking around rivet holes,which can act as a stress concentrator. The apparatus is particularlysuitable for applications where it is desired to image a small area of amuch larger component. The apparatus is lightweight, portable and easyto use. It can readily carried by hand by an operator to be placed whererequired on the object.

The functional blocks illustrated in the figures represent the differentfunctions that the apparatus is configured to perform; they are notintended to define a strict division between physical components in theapparatus. The performance of some functions may be split across anumber of different physical components. One particular component mayperform a number of different functions. The functions may be performedin hardware or software or a combination of the two. The apparatus maycomprise only one physical device or it may comprise a number ofseparate devices. For example, some of the signal processing and imagegeneration may be performed in a portable, hand-held device and some maybe performed in a separate device such as a PC, PDA, phone or tablet. Insome examples, the entirety of the image generation may be performed ina separate device.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

1. An apparatus for imaging structural features below the surface of anobject, the apparatus comprising: a transmitter unit configured totransmit a sound pulse at the object; a receiver unit configured toreceive reflections of sound pulses transmitted by the transmitter unitfrom the object; a signal processing unit configured to: analyse one ormore signals received by the receiver unit from the object; recognise,in the one or more signals, a reflection that was caused by a firststructural feature and a reflection that was caused by a secondstructural feature that is located, in the object, at least partlybehind the first structural feature; and associate each recognisedreflection with a relative depth in the object at which the reflectionoccurred; and an image generation unit configured to generate an imagethat includes a representation of the first and second structuralfeatures in dependence on the recognised reflections and their relativedepths.
 2. An apparatus as claimed in claim 1, the receiver unit beingconfigured to receive a signal comprising multiple reflections of asingle transmitted sound pulse.
 3. An apparatus as claimed in claim 2,the receiver unit being configured to receive the multiple reflectionswhile in a stationary position with respect to the object.
 4. Anapparatus as claimed in claim 2, configured to gate a signal received bythe receiver unit in such a way that the signal passed to the signalprocessing unit for analysis comprises the multiple reflections.
 5. Anapparatus as claimed in claim 1, which is configured to apply anadjustable time-gate to the received signal.
 6. An apparatus as claimedin claim 5, which is configured such that the time-gate is adjustable bya user.
 7. An apparatus as claimed in claim 1, the apparatus comprisinga pulse generation unit configured to form a pulse having a particularshape for transmission as a sound pulse by the transmitter unit.
 8. Anapparatus as claimed in claim 7, the signal processor comprising a matchfilter configured to recognise a pulse having the particular shape inthe received signal.
 9. An apparatus as claimed in claim 7, theapparatus having a plurality of particular pulse shapes available to it,the pulse generation unit being configured to assess an available pulseshape against a performance criterion and select that pulse shape as acandidate for transmission in dependence on that assessment.
 10. Anapparatus as claimed in claim 7, which is configured such that theparticular shape is selectable by the user.
 11. An apparatus as claimedin claim 1, the signal processor being configured to associate eachrecognised reflection with a relative depth that is determined independence on a time that the recognised reflection took to travel fromthe structural feature that caused the reflection to the receiver unit.12. An apparatus as claimed in claim 1, the signal processor beingconfigured to associate each recognised reflection with a maximumamplitude and to adjust that maximum amplitude in dependence on therelative depth associated with that reflection.
 13. An apparatus asclaimed in claim 1, the transmitter unit being configured to transmit aseries of sound pulses into the object and the image generation unitbeing configured to generate an image, for each sound pulse in theseries, in dependence on the reflections of that sound pulse that arerecognised by the signal processing unit.
 14. An apparatus as claimed inclaim 13, the image generation apparatus being configured to generate animage, for a sound pulse in the series, in dependence an image generatedfor a sound pulse that preceded it in the series.
 15. An apparatus asclaimed in claim 1, the image generation apparatus being configured togenerate an image in which the first and second structural features arerepresented at positions in the image that reflect their relative depthsbelow the surface of the object.
 16. An apparatus as claimed in claim 1,the apparatus comprising a handheld device that comprises at least thetransmitter and receiver units.
 17. An apparatus as claimed in claim 1,the apparatus comprising either an integrated display for displaying theimage or being configured to output the image to a handheld displaydevice.
 18. An apparatus as claimed in claim 1, the image generationunit being configured to: identify a reflection having a maximumamplitude that is below a threshold value; and assign a pixel in theimage that corresponds to the identified reflection a predeterminedvalue instead of the reflection's maximum amplitude.
 19. An apparatus asclaimed in claim 19, the image generation unit being configured toassign the pixel a predetermined value that is higher than thereflection's maximum amplitude.
 20. An apparatus as claimed in claim 1,the apparatus comprising a dry coupling medium.
 21. An apparatus asclaimed in claim 1, comprising a matrix array for transmitting andreceiving sound pulses.
 22. A method for imaging structural featuresbelow the surface of an object comprising: transmitting a sound pulse atthe object; receiving, from the object, reflections of sound pulsestransmitted at the object; recognising, in one or more signals receivedfrom the object, a reflection of a transmitted time pulse that wascaused by a first structural feature and a reflection of a transmittedtime pulse that was caused by a second structural feature that islocated, in the object, at least partly behind the first structuralfeature; associating each recognised reflection with a relative depth inthe object at which the reflection occurred; and generating an imagethat includes a representation of the first and second structuralfeatures in dependence on the recognised reflections and their relativedepths.